For altitude-control the Lidar is useful when you want to assure accuracy as much as possible over flat surfaces, as you can already get a (close) estimation of the height thanks to the barometer+GPS. However, the greater the distance you want to measure, the less accurate the Lidar becomes.
This has to do with how the Lidar works, as it simply measures the distance between the first surface it finds in the direction it's pointed at and itself. This can sometimes not be very helpful if you are flying over irregular terrain.
Basically I'd use the Lidar if you want to be (for example) 10M over ANYTHING below you, or similar scenarios, but I'd use the barometer+GPS if you want a constant height.
Take into account the Hokuyo URG-04LX-UG01 (for example) has an accuracy of +-3% when over 1000 meters (as per their specification).